The present invention relates to a numerically controlled drive device comprising a device for detecting operating anomalies intended for detecting accidental collisions, comprising:                at least one motor able to drive a load along at least one path by way of an assembly of transmission elements,        first means of measuring position disposed upstream of at least one of the elements of the said assembly for transmission to the motor and intended to provide a first value of position,        second means of measuring position disposed downstream of this element of the assembly for transmission to said load and intended to provide a second value of position corresponding to the position of the load on said path,        means of calculation for establishing at predetermined intervals error signals corresponding to the difference of the first and second values of position, and        means of analysis for analyzing these error signals so as to detect accidental collisions.        
Such devices have already been used in the machine-tool field as described in document DE 34 26 863 A1.
Comparable drive devices are used in numerous other applications, such as robots, printing machines, equipment for textile production, injection molding machines, stamping and blanking, cranes, satellite antennas, telescopes, etc.
In the drive devices such as represented in FIG. 1, the position of the motor 14 is measured by means of an encoder 15 so as to control the phase commutations at the correct positions of the rotor of the motor. The motor torque is transmitted to the load 18 representing for example the work table or else a spindle fitted with a tool by way of a transmission by pulleys 11 and 13 and belt 12 followed by a ballscrew with nut 17. The position of the load 18 is measured by means of a linear rule 16. The end of travel is determined by a mechanical stop 19 so as to protect the mechanism in case of a defect with the servodrive software. This stop 19 may serve in a machine preparation phase as absolute reference for the initialization of the spindle. Such initialization is necessary in the case where the linear rules do not provide this reference. Generally, end-of-travel contactors are then used for initialization. If these contactors are done away with for cost reduction reasons or the like, then a reliable and effective collision detection device as proposed by the invention becomes necessary in order to use the stop 19 as a reference, without any risk of damaging the mechanics of the device through excessive impact forces.
When using a machine tool, a robot or another item of equipment with an articulation driven by a servomotor, an element in motion may collide with another member of the machine, a workpiece to be machined or with a component of the environment. In this case, the problem to be solved consists in preventing and/or detecting the collision.
The preventive measure that is most widespread within the art, is the limitation of the travel of the spindle in question by means of end-of-travel contactors forming a hard-wired logic limitation, through the prior definition of bounds to the allowable domain of position settings constituting a software limitation, or else with the aid of mechanical stops forming a physical limitation. Within this context of the prevention of collisions, U.S. Pat. No. 5,347,459 has also proposed that the workspace be modeled so as to identify in advance interference between the bodies in relative motion. The drawback of this modeling procedure is the need to ascertain the kinematics of all the objects in the work zone, this information being unavailable for example in regard to a machine tool for the offcuts that could get jammed between the workpiece and the tool. In order to make up for this lack of knowledge regarding the location of bodies in the workspace, recourse has been had for example to vision systems, this nevertheless remaining a complex and expensive solution.
U.S. Pat. No. 4,820,895 proposes an electric touch sensor in the case of an electroerosion machine. Such a device can, according to patent DE 40 08 661 A1, serve in the alignment of a three-dimensional tool on a semi-finished workpiece, doing so through its capacity to detect tool-workpiece collisions accurately and without waste. However, the use of an electric touch sensor requires the presence of electrically conducting objects.
U.S. Pat. No. 5,119,006 has proposed that a certain number of criteria being monitored, such as the absolute value of the servodrive motor torque/current. The procedures described in this patent are restricted to drives with a rigid or very precise transmission, which require mechanical solutions that are needlessly expensive in the case of certain industrial applications for which the introduction of a second rule position sensor makes it possible to implement software procedures for compensating for errors related in particular to flexibility or to homokinetic defects. This compensation may give rise to taxing demands on the motor speed feedback loop, as well as to large speed tracking errors and so to large variations in the motor current. Moreover, if the inertia of the motor dominates the inertia of the load, referred to the motor side, then the effect of the forces of a collision on the motor current is negligible vis-á-vis the forces to be provided for a normal acceleration, respectively deceleration. Thus, the raw signals, such as proposed in U.S. Pat. No. 5,119,006 no longer contain the relevant information making it possible to identify a collision with sufficient sensitivity to avoid waste.
For better utilization for the detection of collision of the motor torque signals, respectively motor current and speed signals, U.S. Pat. No. 5,304,906 proposes the construction of a disturbing torque observer based on the parameters of the drive and the filtering parameters intended to reduce the influence of measurement noise at high frequency. However, the method proposed in this U.S. Pat. No. 5,304,906 is limited to a rigid model for the transmission of the drive. The procedure is thus insufficient for drives for which the flexibility of transmission and the homokinetic defects assume considerable significance. Moreover, the knowledge of the parameters such as the inertia and the torque constant of the motor is incompatible with numerous applications such as machine tools where the mass to be driven may easily vary by a factor of 5 during normal use of the installation.
Patent Application DE 199 60 834 A1 proposes that a rule position be measured on the driven part of the spindle and that this measurement be compared with an indirect measurement taken inside the transmission chain, so as to form an error signal. An abrupt increase in the said signal indicates that a collision or an abnormal incident has occurred. However, nothing is suggested therein other than to compare the temporal increase in the error signal with a limit fixed in advance. The problem with this error signal relating to position between motor and driven part is that it is marred by homokinetic errors, which are characteristic of normal operation and again lead to the triggering limit being fixed at an excessive value. On the other hand, the solution advocated is unsuitable when it is necessary to violently accelerate large inertias. Such is the case for the X and Y spindles of electroerosion machines, which serve to move the assembly of the workpiece to be machined and the tray filled with dielectric liquid, the driven masses possibly reaching as much as one tonne.